Tandem OLED device with intermediate connector

ABSTRACT

A tandem OLED device including an anode; a cathode; at least two electroluminescent units disposed between the anode and the cathode, wherein each of the electroluminescent units includes at least one hole-transporting layer and one organic light-emitting layer; and an intermediate connector disposed between adjacent electroluminescent units, wherein the intermediate connector includes an n-doped organic layer and an electron-accepting layer, the electron-accepting layer being disposed closer to the cathode than the n-doped organic layer, and wherein the n-doped organic layer includes an alkali metal and an organic alkali metal complex.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly assigned U.S. patent application Ser. No.11/393,767, Filed Mar. 30, 2006, now U.S. Published Patent ApplicationNo. 2007/0228938, entitled “Efficient White Light OLED Display withFilters” by T. K. Hatwar et al, U.S. patent application Ser. No.11/259,472 Filed Oct. 26, 2005, now U.S. Published Patent ApplicationNo. 2006/0286405, entitled “Organic Element for Low VoltageElectroluminescent Devices” by William J. Begley et al, and U.S. patentapplication Ser. No. 12/023,216 filed Jan. 31, 2008, now U.S. Pat. No.7,821,201, entitled “Tandem OLED Device With Intermediate Connector” byT. K. Hatwar et al, the disclosures of which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to tandem OLED devices and to intermediateconnectors between them.

BACKGROUND OF THE INVENTION

An organic light-emitting diode device, also called an OLED, commonlyincludes an anode, a cathode, and an organic electroluminescent (EL)unit sandwiched between the anode and the cathode. The organic EL unitcommonly includes a hole-transporting layer (HTL), a light-emittinglayer (LEL), and an electron-transporting layer (ETL). OLEDs areattractive because of their low drive voltage, high luminance, wideviewing-angle, and capability for full color displays and for otherapplications. Tang et al. described this multilayer OLED in their U.S.Pat. Nos. 4,769,292 and 4,885,211.

OLEDs can emit different colors, such as red, green, blue, or white,depending on the emitting property of its LEL. Recently, there is anincreasing demand for broadband OLEDs to be incorporated into variousapplications, such as a solid-state lighting source, color display, or afull color display. By broadband emission, it is meant that an OLEDemits sufficiently broad light throughout the visible spectrum so thatsuch light can be used in conjunction with filters or color changemodules to produce displays with at least two different colors or a fullcolor display. In particular, there is a need forbroadband-light-emitting OLEDs (or broadband OLEDs) where there issubstantial emission in the red, green, and blue portions of thespectrum, i.e., a white light-emitting OLED (white OLED). The use ofwhite OLEDs with color filters provides a simpler manufacturing processthan an OLED having separately patterned red, green, and blue emitters.This can result in higher throughput, increased yield, and cost savings.White OLEDs have been reported, e.g. by Kido et al. in Applied PhysicsLetters, 64, 815 (1994), J. Shi et al. in U.S. Pat. No. 5,683,823, Satoet al. in JP 07-142169, Deshpande et al. in Applied Physics Letters, 75,888 (1999), and Tokito, et al. in Applied Physics Letters, 83, 2459(2003).

In order to achieve broadband emission from an OLED, more than one typeof molecule has to be excited, because each type of molecule only emitslight with a relatively narrow spectrum under normal conditions. Alight-emitting layer having a host material and one or more luminescentdopant(s) can achieve light emission from both the host and thedopant(s), resulting in a broadband emission in the visible spectrum ifthe energy transfer from the host material to the dopant(s) isincomplete. To achieve a white OLED having a single light-emittinglayer, the concentrations of light-emitting dopants must be carefullycontrolled. This produces manufacturing difficulties. A white OLEDhaving two or more light-emitting layers can have better color andbetter luminance efficiency than a device with one light-emitting layer,and the variability tolerance for dopant concentration is higher. It hasalso been found that white OLEDs having two light-emitting layers aretypically more stable than OLEDs having a single light-emitting layer.However, it is difficult to achieve light emission with strong intensityin the red, green, and blue portions of the spectrum. A white OLED withtwo light-emitting layers typically has two intensive emission peaks.

A tandem OLED structure (sometimes called a stacked OLED or a cascadedOLED) has been disclosed by Jones et al. in U.S. Pat. No. 6,337,492,Tanaka et al. in U.S. Pat. No. 6,107,734, Kido et al. in JP PatentPublication 2003/045676A and U.S. Published Patent Application No.2003/0189401 A1, and Liao et al. in U.S. Pat. No. 6,717,358 and U.S.Patent Application Publication No. 2003/0170491 A1. This tandem OLED isfabricated by stacking several individual OLED units vertically anddriving the stack using a single power source. The advantage is thatluminance efficiency, lifetime, or both are increased. However, thetandem structure increases the driving voltage approximately inproportion to the number of OLED units stacked together.

Matsumoto and Kido et al. reported in SID 03 Digest, 979 (2003) that atandem white OLED is constructed by connecting a greenish-blue EL unitand an orange EL unit in the device, and white light emission isachieved by driving this device with a single power source. Althoughluminance efficiency is increased, this tandem white OLED device hasweaker green and red color components in the spectrum. In U.S. PatentApplication Publication No. 2003/0170491 A1, Liao et al. describe atandem white OLED structure by connecting a red EL unit, a green ELunit, and a blue EL unit in series within the device. When the tandemwhite OLED is driven by a single power source, white light emission isformed by spectral combination from the red, green, and blue EL units.

Notwithstanding these developments, there remains a need to improveefficiency and driving voltage of tandem OLED devices while maintaininggood broadband emission.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an OLEDdevice with improved efficiency and luminance stability.

This object is achieved by a tandem OLED device comprising:

(a) an anode;

(b) a cathode;

(c) at least two electroluminescent units disposed between the anode andthe cathode, wherein each of the electroluminescent units includes atleast one hole-transporting layer and one organic light-emitting layer;and

(d) an intermediate connector disposed between adjacentelectroluminescent units, wherein the intermediate connector includes ann-doped organic layer and an electron-accepting layer, theelectron-accepting layer being disposed closer to the cathode than then-doped organic layer, and wherein the n-doped organic layer includes analkali metal and an organic alkali metal complex.

It is an advantage of this invention that it can provide improvedefficiency, or lower drive voltage, or both.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional diagram of one embodiment of a tandemOLED device according to this invention; and

FIG. 2 shows a cross-sectional diagram of another embodiment of a tandemOLED device according to this invention.

Since device feature dimensions such as layer thicknesses are frequentlyin sub-micrometer ranges, the drawings are scaled for ease ofvisualization rather than dimension accuracy.

DETAILED DESCRIPTION OF THE INVENTION

The term “OLED device” is used in its art-recognized meaning of adisplay device including organic light-emitting diodes as pixels. It canmean a device having a single pixel. The term “tandem OLED device” isused in its art-recognized meaning of a display device including atleast two OLED device units stacked vertically and powered by a singlepower source. The term “OLED display” as used herein means an OLEDdevice including a plurality of pixels, which can be of differentcolors. A color OLED device emits light of at least one color. The term“pixel” is employed in its art-recognized usage to designate an area ofa display panel that is stimulated to emit light independently of otherareas. It is recognized that in full color systems, several pixels ofdifferent colors will be used together to produce a wide range ofcolors, and a viewer can term such a group a single pixel. For thepurposes of this discussion, such a group will be considered severaldifferent colored pixels. In accordance with this disclosure, broadbandemission is light that has significant components in multiple portionsof the visible spectrum, for example, blue and green. Broadband emissioncan also include the situation where light is emitted in the red, green,and blue portions of the spectrum in order to produce white light. Whitelight is that light that is perceived by a user as having a white color,or light that has an emission spectrum sufficient to be used incombination with color filters to produce a practical full colordisplay. The term “white light-emitting” as used herein refers to adevice that produces white light internally, even though part of suchlight can be removed by color filters before viewing.

Turning now to FIG. 1, there is shown a cross-sectional view of a pixelof a tandem white-light-emitting OLED device 10 according to oneembodiment of the present invention. OLED device 10 includes a substrate20, two spaced electrodes, which are an anode 30 and a cathode 90, atleast two electroluminescent units 70 and 75 disposed between theelectrodes, and an intermediate connector 80 disposed between adjacentelectroluminescent units 70 and 75. Hatwar et al. in above-cited U.S.patent application Ser. No. 11/393,767 has described the use of multipleelectroluminescent units of this arrangement. Each electroluminescentunit includes at least one hole-transporting layer, e.g.hole-transporting layers 40 and 45. Electroluminescent unit 70, which isclosest to anode 30, can also include a hole-injecting layer 35.Electroluminescent unit 75, which is closest to cathode 90, can alsoinclude an electron-transporting layer 55. Each electroluminescent unitincludes at least one organic light-emitting layer. Electroluminescentunits 70 and 75 each produce different emission spectra, but that is notrequired for this invention. In this embodiment, electroluminescent unit70 includes a blue light-emitting layer 50 b that includes a bluelight-emitting compound and a yellow light-emitting layer 50 y thatincludes a yellow light-emitting compound. As used herein, the term“yellow light-emitting compound” refers to a substance that has itsprimary light emission in the yellow to red region, that is, from about570 nm to 700 nm. In this embodiment, electroluminescent unit 75includes a green light-emitting layer 51 g that includes a greenlight-emitting compound and produces green emission, red light-emittinglayer 51 r that includes a red light-emitting compound and produces redemission, and blue light-emitting layer 51 b that includes a bluelight-emitting compound and produces blue emission. The inventiondescribed herein is not limited to this combination of light-emittinglayers, and many different light-emitting layers and combinations oflight-emitting layers can be used as known to those skilled in the art.

Tandem OLED device 10 further includes an intermediate connector 80disposed between adjacent electroluminescent units 70 and 75. Theintermediate connector 80 provides effective carrier injection into theadjacent electroluminescent units. Metals, metal compounds, or otherinorganic compounds are effective for carrier injection. However, suchmaterials often have low resistivity, which can result in pixelcrosstalk. Also, the optical transparency of the layers constitutingintermediate connector 80 should be as high as possible to permitradiation produced in the electroluminescent units to exit the device.Therefore, it is often preferred to use mainly organic materials in theintermediate connector 80. Intermediate connector 80 includes an n-dopedorganic layer 95 and an electron-accepting layer 38. Electron-acceptinglayer 38 is disposed closer to cathode 90 than is n-doped organic layer95.

In intermediate connector 80, n-doped organic layer 95 includes analkali metal and an organic alkali metal complex. The alkali metal ofthe organic alkali metal complex belongs to Group 1 of the periodictable. Of these, lithium is highly preferred. Organic alkali metalcomplexes useful in this invention include organic lithium compoundsaccording to Formula A:(Li⁺)_(m)(Q)_(n)  Awherein:

Q is an anionic organic ligand; and

m and n are independently selected integers selected to provide aneutral charge on the complex.

The anionic organic ligand Q is most suitably monoanionic and containsat least one ionizable site consisting of oxygen, nitrogen or carbon. Inthe case of enolates or other tautomeric systems containing oxygen, itwill be considered and drawn with the lithium bonded to the oxygen,although the lithium can be bonded elsewhere to form a chelate. It isalso desirable that the ligand contains at least one nitrogen atom thatcan form a coordinate or dative bond with the lithium. The integers mand n can be greater than 1 reflecting a known propensity for someorganic lithium compounds to form cluster complexes.

In another embodiment, Formula B represents the organic alkali metalcomplex:

wherein:

Z and the dashed arc represent two to four atoms and the bonds necessaryto complete a 5- to 7-membered ring with the lithium cation;

each A represents hydrogen or a substituent and each B representshydrogen or an independently selected substituent on the Z atoms,provided that two or more substituents can combine to form a fused ringor a fused ring system;

j is 0-3 and k is 1 or 2; and

m and n are independently selected integers selected to provide aneutral charge on the complex.

Of compounds of Formula B, it is most desirable that the A and Bsubstituents together form an additional ring system. This additionalring system can further contain additional heteroatoms to form amultidentate ligand with coordinate or dative bonding to the lithium.Desirable heteroatoms are nitrogen or oxygen. Useful compounds ofFormula B have been described by Begley in U.S. Patent ApplicationPublication No. 2006/086405, the disclosure of which is hereinincorporated by reference. In Formula B, it is preferred that the oxygenshown is part of a hydroxyl, carboxy or keto group. Examples of suitablenitrogen ligands are 8-hydroxyquinoline, 2-hydroxymethylpyridine,pipecolinic acid or 2-pyridinecarboxylic acid.

Specific examples of useful organic alkali metal complexes are asfollows:

Particularly useful of the above are phenanthroline derivatives, e.g. C1and related complexes. Other examples are described by Begley.

The alkali metal in n-doped organic layer 95 is preferably lithium. Theconcentration of Li metal is about 0.1 to 5% by volume of the hostmaterial composition, and preferably within the 0.5 to 2% range.

In some embodiments, n-doped organic layer 95 can further include anuncharged phenanthroline derivative. Such phenanthroline derivatives areknown to be useful in electron-transporting layers. A useful unchargedphenanthroline derivative is 4,7-diphenyl-1,10-phenanthroline, alsoknown as bathophen or Bphen.

Electron-accepting layer 38 can include an inorganic compound(s), suchas metal oxide, metal nitride, metal carbide, a complex of a metal ionand organic ligands, and a complex of a transition metal ion and organicligands. Suitable materials for use in electron-accepting layer 38 canalso include plasma-deposited fluorocarbon polymers (CFx) as describedin U.S. Pat. No. 6,208,075, a strong oxidizing agent such as ahexaazatriphenylene derivative (Structure D) as described in U.S. Pat.No. 6,720,573 B2 and in U.S. Published Patent Application No.2004/113547 A1, or another strong oxidizing agent, which can be organic,such as F₄TCNQ or inorganic, such as molybdenum oxide, FeCl₃, or FeF₃.Other useful oxidizing agents, also known as electron-acceptingmaterials, are described in commonly-assigned U.S. Ser. No. 11/867,707by Liao et al., the disclosure of which is incorporated herein byreference.

A useful hexaazatriphenylene derivative for this purpose ishexacyanohexaazatriphenylene, which has the structure:

While not necessary, electron-accepting layer 38 can also be composed oftwo components: for example, one of the organic materials mentionedabove, such as an amine compound, doped with a strong oxidizing agent,such as dipyrazino[2,3-f:2′,3′-h]quinoxalinehexacarbonitrile, F₄TCNQ, orFeCl₃.

OLED devices are commonly formed on a supporting substrate, e.g. OLEDsubstrate 20. The electrode in contact with the substrate isconveniently referred to as the bottom electrode. The substrate 20 caneither be light transmissive or opaque, depending on the intendeddirection of light emission. The light transmissive property isdesirable for viewing the EL emission through the substrate. Transparentglass or plastic is commonly employed in such cases. For applicationswhere the EL emission is viewed through the top electrode, thetransmissive characteristic of the bottom support is immaterial, andtherefore can be light transmissive, light-absorbing, or lightreflective. Substrates for use in this case include, but are not limitedto, glass, plastic, semiconductor materials, silicon, ceramics, andcircuit board materials. Of course, it is necessary to provide in thesedevice configurations a light-transparent top electrode.

An anode 30 is formed over substrate 20. When EL emission is viewedthrough the anode, the anode should be transparent, or substantiallytransparent, to the emission of interest. Common transparent anodematerials used in the present invention are indium-tin oxide (ITO),indium-zinc oxide (IZO), and tin oxide, but other metal oxides can workincluding, but not limited to, aluminum- or indium-doped zinc oxide,magnesium-indium oxide, and nickel-tungsten oxide. In addition to theseoxides, metal nitrides such as gallium nitride, and metal selenides suchas zinc selenide, and metal sulfides such as zinc sulfide, can be usedas the anode. For applications where EL emission is viewed only throughthe cathode electrode, the transmissive characteristics of the anode areimmaterial and many conductive materials can be used, regardless iftransparent, opaque, or reflective. Example conductors for the presentinvention include, but are not limited to, gold, iridium, molybdenum,palladium, and platinum. Typical anode materials, transmissive orotherwise, have a work function no less than 4.0 eV. Desired anodematerials can be deposited by any suitable process such as evaporation,sputtering, chemical vapor deposition, or electrochemical deposition. Ifnecessary, anode materials can be patterned using well-knownphotolithographic processes.

A cathode 90 is formed over the other OLED layers. If the device istop-emitting, the electrode must be transparent or nearly transparent.For such applications, metals must be thin (preferably less than 25 nm)or one must use transparent conductive oxides (e.g. indium-tin oxide,indium-zinc oxide), or a combination of these materials. Opticallytransparent cathodes have been described in more detail in U.S. Pat. No.5,776,623. If the device is bottom-emitting, the cathode can be anyconductive material known to be useful in OLED devices, including metalssuch as aluminum, molybdenum, gold, iridium, silver, magnesium, theabove transparent conductive oxides, or combinations of these. Desirablematerials promote electron injection at low voltage and have effectivestability. Useful cathode materials often contain a low work functionmetal (<4.0 eV) or metal alloy. Cathode materials can be deposited byevaporation, sputtering, or chemical vapor deposition. When needed,patterning can be achieved through many well known methods including,but not limited to, through-mask deposition, integral shadow masking asdescribed in U.S. Pat. No. 5,276,380 and EP 0 732 868, laser ablation,and selective chemical vapor deposition.

Hole-injecting layer 35 can be formed of a single or a mixture oforganic or inorganic materials. The hole-injecting layer can be dividedinto several layers, with each layer being composed of either the sameor different materials. The hole-injecting material can serve to improvethe film formation property of subsequent organic layers and tofacilitate injection of holes into the hole-transporting layer. Suitablematerials for use in the hole-injecting layer are electron-deficientmaterials that include, but are not limited to porphyrin andphthalocyanine compounds as described in U.S. Pat. No. 4,720,432,phosphazine compounds, and certain aromatic amine compounds which arestronger electron donors than conventional hole-transporting layermaterials, such as N,N,N,N-tetraarylbenzidine compounds. In anotheruseful embodiment, the hole-injecting layer includes a compoundincorporating a para-phenylenediamine as taught in EP 0 891 121 A1 andEP 1 029 909 A1, dihydrophenazine, 2,6-diaminonaphthalene,2,6-diaminoanthracene, 2,6,9,10-tetraminoanthracene, anilinoethylene,N,N,N,N-tetraarylbenzidine, mono- or polyaminated perylene, mono- orpolyaminated coronene, polyaminated pyrene, mono- or polyaminatedfluoranthene, mono- or polyaminated chrysene, mono- or polyaminatedanthanthrene, mono- or polyaminated triphenylene, or mono- orpolyaminated tetracene moiety while the second material includes anamine compound that contains either a N,N,N,N-tetraarylbenzidine or aN-arylcarbazole moiety.

The hole-injecting layer 35 can include an inorganic compound(s), suchas metal oxide, metal nitride, metal carbide, a complex of a metal ionand organic ligands, and a complex of a transition metal ion and organicligands.

Suitable materials for use in the hole-injecting layer can also includeplasma-deposited fluorocarbon polymers (CFx) as described in U.S. Pat.No. 6,208,075, a strong oxidizing agent such as a hexaazatriphenylenederivative (Structure D, above) as described in U.S. Pat. No. 6,720,573B2 and in U.S. Published Patent Application No. 2004/113547 A1, oranother strong oxidizing agent, which can be organic, such as F₄TCNQ orinorganic, such as molybdenum oxide, FeCl₃, or FeF₃. A usefulhexaazatriphenylene derivative for this purpose ishexacyanohexaazatriphenylene (Structure E, above).

While not necessary, the hole-injecting layer can also be composed oftwo components: for example, one of the organic materials mentionedabove, such as an amine compound, doped with a strong oxidizing agent,such as dipyrazino[2,3-f:2′,3′-h]quinoxalinehexacarbonitrile, F₄TCNQ, orFeCl₃.

Hole-transporting layer 40 can include any hole-transporting materialuseful in OLED devices, many examples of which are known to thoseskilled in the art. Desired hole-transporting materials can be depositedby any suitable way such as evaporation, sputtering, chemical vapordeposition, electrochemical process, thermal transfer, or laser thermaltransfer from a donor material. Hole-transporting materials useful inhole-transporting layers are well known to include compounds such as anaromatic tertiary amine, where the latter is understood to be a compoundcontaining at least one trivalent nitrogen atom that is bonded only tocarbon atoms, at least one of which is a member of an aromatic ring. Inone form the aromatic tertiary amine can be an arylamine, such as amonoarylamine, diarylamine, triarylamine, or a polymeric arylamine.Exemplary monomeric triarylamines are illustrated by Klupfel et al. inU.S. Pat. No. 3,180,730. Other suitable triarylamines substituted withone or more vinyl radicals and/or including at least one activehydrogen-containing group are disclosed by Brantley et al. in U.S. Pat.Nos. 3,567,450 and 3,658,520.

A more preferred class of aromatic tertiary amines are those whichinclude at least two aromatic tertiary amine moieties as described inU.S. Pat. Nos. 4,720,432 and 5,061,569. Such compounds include thoserepresented by structural Formula F.

wherein:

Q₁ and Q₂ are independently selected aromatic tertiary amine moieties;and

G is a linking group such as an arylene, cycloalkylene, or alkylenegroup of a carbon to carbon bond.

One class of such aromatic tertiary amines are the tetraaryldiamines.Desirable tetraaryldiamines include two diarylamino groups linkedthrough an arylene group. Useful tetraaryldiamines include thoserepresented by Formula G.

wherein:

each Are is an independently selected arylene group, such as a phenyleneor anthracene moiety;

n is an integer of from 1 to 4; and

Ar, R₇, R₈, and R₉ are independently selected aryl groups.

The various alkyl, alkylene, aryl, and arylene moieties of the foregoingstructural Formulae F and G can each in turn be substituted. Typicalsubstituents include alkyl groups, alkoxy groups, aryl groups, aryloxygroups, and halogens such as fluoride, chloride, and bromide. Thevarious alkyl and alkylene moieties typically contain from 1 to about 6carbon atoms. The cycloalkyl moieties can contain from 3 to about 10carbon atoms, but typically contain five, six, or seven carbonatoms—e.g., cyclopentyl, cyclohexyl, and cycloheptyl ring structures.The aryl and arylene moieties are usually phenyl and phenylene moieties.Usefully, the hole-transporting host material is anN,N,N′,N′-tetraarylbenzidine, wherein the Are of Formula G represents aphenylene group and n equals 2.

Electron-transporting layer 55 can include any electron-transportingmaterial useful in OLED devices, many examples of which are known tothose skilled in the art. Electron-transporting layer 55 can contain oneor more metal chelated oxinoid compounds, including chelates of oxineitself, also commonly referred to as 8-quinolinol or 8-hydroxyquinoline.Such compounds help to inject and transport electrons and exhibit bothhigh levels of performance and are readily fabricated in the form ofthin films. Exemplary of contemplated oxinoid compounds are thosesatisfying structural Formula H.

wherein:

M represents a metal;

n is an integer of from 1 to 3; and

Z independently in each occurrence represents the atoms completing anucleus having at least two fused aromatic rings.

From the foregoing it is apparent that the metal can be a monovalent,divalent, or trivalent metal. The metal can, for example, be an alkalimetal, such as lithium, sodium, or potassium; an alkaline earth metal,such as magnesium or calcium; or an earth metal, such as boron oraluminum. Generally any monovalent, divalent, or trivalent metal knownto be a useful chelating metal can be employed.

Z completes a heterocyclic nucleus containing at least two fusedaromatic rings, at least one of which is an azole or azine ring.Additional rings, including both aliphatic and aromatic rings, can befused with the two required rings, if required. To avoid addingmolecular bulk without improving on function the number of ring atoms isusually maintained at 18 or less.

Illustrative of useful chelated oxinoid compounds are the following:

CO-1: Aluminum trisoxine [alias, tris(8-quinolinolato)aluminum(III)];

CO-2: Magnesium bisoxine [alias, bis(8-quinolinolato)magnesium(II)];

CO-3: Bis[benzo {f}-8-quinolinolato]zinc (II);

CO-4:Bis(2-methyl-8-quinolinolato)aluminum(III)-μ-oxo-bis(2-methyl-8-quinolinolato)aluminum(III);

CO-5: Indium trisoxine [alias, tris(8-quinolinolato)indium];

CO-6: Aluminum tris(5-methyloxine) [alias,tris(5-methyl-8-quinolinolato) aluminum(III)];

CO-7: Lithium oxine [alias, (8-quinolinolato)lithium(I)];

CO-8: Gallium oxine [alias, tris(8-quinolinolato)gallium(III)]; and

CO-9: Zirconium oxine [alias, tetra(8-quinolinolato)zirconium(IV)].

Other electron-transporting materials include various butadienederivatives as disclosed in U.S. Pat. No. 4,356,429 and variousheterocyclic optical brighteners as described in U.S. Pat. No.4,539,507. Benzazoles, oxadiazoles, triazoles, pyridinethiadiazoles,triazines, phenanthroline derivatives, and some silole derivatives arealso useful electron-transporting materials. Substituted1,10-phenanthroline compounds known to be useful aselectron-transporting materials are disclosed in JP2003/115387;JP2004/311184; JP2001/267080; and WO2002/043449.

The embodiment shown herein includes five organic light-emitting layers:blue light-emitting layers 50 b and 51 b, yellow light-emitting layer 50y, green light-emitting layer 51 g, and red light-emitting layer 51 r.However, this invention is not limited to this configuration. A largevariety of light-emitting layers are known in the art and can be used inthis invention. Such light-emitting layer can include red light-emittinglayers, yellow light-emitting layers, green light-emitting layers, bluelight-emitting layers, or combinations of these. Light-emitting layerssuch as those described herein produce light in response tohole-electron recombination. Desired organic light-emitting materialscan be deposited by any suitable way such as evaporation, sputtering,chemical vapor deposition, electrochemical process, or radiation thermaltransfer from a donor material. The light-emitting layers in thisinvention include one or more host materials doped with one or morelight-emitting guest compounds or dopants where light emission comesprimarily from the dopant. A dopant is selected to produce color lighthaving a particular spectrum and to have other desirable properties.Dopants are typically coated as 0.01 to 15% by weight into the hostmaterial.

A light-emitting layer can include an anthracene host, desirably a9,10-diarylanthracene, certain derivatives of which (Formula I) areknown to constitute a class of useful host materials capable ofsupporting electroluminescence, and are particularly suitable for lightemission of wavelengths longer than 400 nm, e.g., blue, green, yellow,orange or red:

wherein R¹, R², R³, and R⁴ represent one or more substituents on eachring where each substituent is individually selected from the followinggroups:

Group 1: hydrogen, or alkyl of from 1 to 24 carbon atoms;

Group 2: aryl or substituted aryl of from 5 to 20 carbon atoms;

Group 3: carbon atoms from 4 to 24 necessary to complete a fusedaromatic ring of anthracenyl, pyrenyl, or perylenyl;

Group 4: heteroaryl or substituted heteroaryl of from 5 to 24 carbonatoms as necessary to complete a fused heteroaromatic ring of furyl,thienyl, pyridyl, quinolinyl or other heterocyclic systems;

Group 5: alkoxylamino, alkylamino, or arylamino of from 1 to 24 carbonatoms; and

Group 6: fluorine or cyano.

Particularly useful are compounds wherein R¹ and R², and in some casesR³ and R⁴, represent additional aromatic rings.

Also useful as host or co-host materials are certain hole-transportingmaterials such as aromatic tertiary amines (e.g. Structures F and G,above) and certain electron-transporting materials such as chelatedoxinoid compounds (e.g. Structure H, above).

In addition to a host material as described above, the light-emittinglayers also include one or more dopants as the first light-emittingmaterial. A red-light-emitting dopant can include a diindenoperylenecompound of the following structure J:

wherein:

-   -   X₁-X₁₆ are independently selected as hydrogen or substituents        that include alkyl groups of from 1 to 24 carbon atoms; aryl or        substituted aryl groups of from 5 to 20 carbon atoms;        hydrocarbon groups containing 4 to 24 carbon atoms that complete        one or more fused aromatic rings or ring systems; or halogen,        provided that the substituents are selected to provide an        emission maximum between 560 nm and 640 nm.

Illustrative examples of useful red dopants of this class are shown byHatwar et al. in U.S. Pat. No. 7,247,394, the disclosure of which isincorporated by reference.

Some other red dopants belong to the DCM class of dyes represented byFormula K:

wherein Y₁-Y₅ represent one or more groups independently selected from:hydro, alkyl, substituted alkyl, aryl, or substituted aryl; Y₁-Y₅independently include acyclic groups or can be joined pairwise to formone or more fused rings; provided that Y₃ and Y₅ do not together form afused ring. Structures of particularly useful dopants of the DCM classare shown by Ricks et al. in U.S. Pat. No. 7,252,893, the disclosure ofwhich is incorporated by reference.

A light-emitting yellow dopant can include a compound of the followingstructures:

wherein A₁-A₆ and A′₁-A′₆ represent one or more substituents on eachring and where each substituent is individually selected from one of thefollowing:

-   -   Category 1: hydrogen, or alkyl of from 1 to 24 carbon atoms;    -   Category 2: aryl or substituted aryl of from 5 to 20 carbon        atoms;    -   Category 3: hydrocarbon containing 4 to 24 carbon atoms,        completing a fused aromatic ring or ring system;    -   Category 4: heteroaryl or substituted heteroaryl of from 5 to 24        carbon atoms such as thiazolyl, furyl, thienyl, pyridyl,        quinolinyl or other heterocyclic systems, which are bonded via a        single bond, or complete a fused heteroaromatic ring system;    -   Category 5: alkoxylamino, alkylamino, or arylamino of from 1 to        24 carbon atoms; or    -   Category 6: fluoro or cyano.

Examples of particularly useful yellow dopants are shown by Ricks et al.in U.S. Pat. No. 7,252,893.

A green light-emitting dopant can include a quinacridone compound, e.g.a compound of the following structure:

wherein substituent groups R₁ and R₂ are independently alkyl, alkoxyl,aryl, or heteroaryl; and substituent groups R₃ through R₁₂ areindependently hydrogen, alkyl, alkoxyl, halogen, aryl, or heteroaryl,and adjacent substituent groups R₃ through R₁₀ can optionally beconnected to form one or more ring systems, including fused aromatic andfused heteroaromatic rings, provided that the substituents are selectedto provide an emission maximum between 510 nm and 540 nm. Alkyl,alkoxyl, aryl, heteroaryl, fused aromatic ring and fused heteroaromaticring substituent groups can be further substituted. Some examples ofuseful quinacridones include those disclosed in U.S. Pat. No. 5,593,788and in U.S. Published Patent Application No. 2004/0001969A1.

Examples of useful quinacridone green dopants include:

A green light-emitting dopant can also include a 2,6-diaminoanthracenelight-emitting dopant, as represented by the formula below:

wherein d₁, d₃-d₅, and d₇-d₁₀ can be the same or different and eachrepresents hydrogen or an independently selected substituent, and each hcan be the same or different and each represents one or moreindependently selected substituents, provided that two substituents cancombine to form a ring group and a-d are independently 0-5.

A blue-light-emitting dopant can include a bis(azinyl)azene boroncomplex compound of the structure P:

wherein:

-   -   A and A′ represent independent azine ring systems corresponding        to 6-membered aromatic ring systems containing at least one        nitrogen;    -   (X^(a))_(n) and (X^(b))_(m) represent one or more independently        selected substituents and include acyclic substituents or are        joined to form a ring fused to A or A′;    -   m and n are independently 0 to 4;    -   Z^(a) and Z^(b) are independently selected substituents;    -   1, 2, 3, 4, 1′, 2′, 3′, and 4′ are independently selected as        either carbon or nitrogen atoms; and    -   provided that X^(a), X^(b), Z^(a), and Z^(b), 1, 2, 3, 4, 1′,        2′, 3′, and 4′ are selected to provide blue luminescence.

Some examples of the above class of dopants are disclosed by Ricks etal. The concentration of this class of dopants in a light-emitting layeris desirably between 0.1% and 5%.

Another class of blue dopants is the perylene class. Particularly usefulblue dopants of the perylene class include perylene andtetra-t-butylperylene (TBP).

Another class of blue dopants includes blue-emitting derivatives of suchstyrylarenes and distyrylarenes as distyrylbenzene, styrylbiphenyl, anddistyrylbiphenyl, including compounds described in U.S. Pat. No.5,121,029, and U.S. Published Patent Application No. 2006/0093856 byHelber et al. Among such derivatives that provide blue luminescence,particularly useful in second light-emitting layer 52 are thosesubstituted with diarylamino groups and herein termed aminostyrylarenedopants. Examples includebis[2-[4-[N,N-diarylamino]phenyl]vinyl]-benzenes of the generalstructure Q1 shown below:

[N,N-diarylamino][2-[4-[N,N-diarylamino]phenyl]vinyl]biphenyls of thegeneral structure Q2 shown below:

and bis[2-[4-[N,N-diarylamino]phenyl]vinyl]biphenyls of the generalstructure Q3 shown below:

In Formulas Q1 to Q3, X₁-X₄ can be the same or different, andindividually represent one or more substituents such as alkyl, aryl,fused aryl, halo, or cyano. In a preferred embodiment, X₁-X₄ areindividually alkyl groups, each containing from one to about ten carbonatoms. A particularly preferred blue dopant of this class is disclosedby Ricks et al in U.S. Pat. No. 7,252,893.

OLED device 10 can include other layers as well. For example, anelectron-injecting layer, such as alkaline or alkaline earth metals,alkali halide salts, or alkaline or alkaline earth metal doped organiclayers, can also be present between cathode 90 and electron-transportinglayer 55.

Turning now to FIG. 2, there is shown a cross-sectional view of anotherembodiment of a tandem OLED device 15 in accordance with this invention.In this embodiment, an intermediate connector 85 further includesundoped layer 65, which is in contact with n-doped layer 95, and whichis closer to anode 30 than to cathode 90. An undoped layer 65 includesan aromatic hydrocarbon derivative, desirably an anthracene derivativeas described above for Structure I. Undoped layer 65 can also include anorganic alkali metal complex, as described above for Structure A. Anexample of an organic alkali metal complex useful in undoped layer 65 isa salt of hydroxyquinoline, e.g. 8-quinolinolato lithium:

The invention and its advantages can be better appreciated by thefollowing comparative examples. Examples 3 to 6 are representativeexamples of this invention, while Examples 1 and 2 are non-inventivetandem OLED examples for comparison purposes. The layers described asvacuum-deposited were deposited by evaporation from heated boats under avacuum of approximately 10⁻⁶ Torr. After deposition of the OLED layerseach device was then transferred to a dry box for encapsulation. TheOLED has an emission area of 10 mm². The devices were tested by applyinga current of 20 mA/cm² across electrodes. The results from Examples 1 to6 are given in Table 1.

TABLE 1 Device data measured at 20 mA/cm² Lum Power Luminance EfficiencyEfficiency Device # (cd/m²) Voltage (cd/A) (W/A) CIEx CIEy lm/W QE %Example 1 2955 9.3 14.8 0.150 0.312 0.346 5.0 6.6 (Comparative) Example2 3005 9.2 15.0 0.153 0.311 0.346 5.1 6.7 (Comparative) Example 3 30728.6 15.3 0.149 0.327 0.363 5.6 6.6 (Inventive) Example 4 3036 8.8 15.20.148 0.320 0.361 5.5 6.5 (Inventive) Example 5 3446 9.1 17.2 0.1640.333 0.374 6.0 7.3 (Inventive) Example 6 3377 8.9 16.9 0.165 0.3280.359 6.0 7.3 (Inventive)

Example 1 (Comparative)

-   -   1. A clean glass substrate was deposited by sputtering with        indium tin oxide (ITO) to form a transparent electrode of 60 nm        thickness.    -   2. The above-prepared ITO surface was treated with a plasma        oxygen etch.    -   3. The above-prepared substrate was further treated by        vacuum-depositing a 10 nm layer of hexacyanohexaazatriphenylene        (CHATP) as a hole-injecting layer (HIL).

-   -   4. The above-prepared substrate was further treated by        vacuum-depositing a 150 nm layer of        4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (NPB) as a        hole-transporting layer (HTL).    -   5. The above-prepared substrate was further treated by        vacuum-depositing a 20 nm yellow light-emitting layer including        48% NPB (as host) and 48%        9-(1-naphthyl)-10-(2-naphthyl)anthracene (NNA) as a co-host with        4% yellow-orange emitting dopant diphenyltetra-t-butylrubrene        (PTBR).

-   -   6. The above-prepared substrate was further treated by        vacuum-depositing a 30 nm blue light-emitting layer including        91% NNA host and 8% NPB co-host with 1% BEP as blue-emitting        dopant.

-   -   7. A 40 nm n-doped organic layer was vacuum-deposited, including        49% 4,7-diphenyl-1,10-phenanthroline (also known as bathophen or        Bphen), 49% tris(8-quinolinolato)aluminum (III) (ALQ) as        co-host, with 2% Li metal.    -   8. The above-prepared substrate was further treated by        vacuum-depositing a 10 nm layer of CHATP as an        electron-accepting layer.    -   9. The above-prepared substrate was further treated by        vacuum-depositing a 10 nm layer of NPB as a hole-transporting        layer (HTL).    -   10. The above-prepared substrate was further treated by        vacuum-depositing a 20 nm red light-emitting layer including        94.5% of NPB and 5% PTBR as a yellow-emitting dopant with 0.5%        dibenzo{[f,f′]-4,4′7,7′-tetraphenyl]diindeno-[1,2,3-cd:1′,2′,3′-lm]perylene        (TPDBP) as a red emitting dopant.    -   11. The above-prepared substrate was further treated by        vacuum-depositing a 15 nm green light-emitting layer including        84.4% 2-phenyl-9,10-bis(2-naphthyl)anthracene (PBNA), 15% NPB,        and 0.6% diphenylquinacridone (DPQ) as green emitting dopant.    -   12. The above-prepared substrate was further treated by        vacuum-depositing a 20 nm blue light-emitting layer including        99% PBNA host with 1% BEP as blue-emitting dopant.    -   13. A 35 nm mixed electron-transporting layer was        vacuum-deposited, including 49% Bphen, 49% ALQ as co-host, with        2% Li metal.    -   14. A 100 nm layer of aluminum was evaporatively deposited onto        the substrate to form a cathode layer.

Example 2 (Comparative)

An OLED device was constructed as described above for Example 1 exceptthat Step 7 was as follows:

-   -   7. A 40 nm n-doped organic layer was vacuum-deposited, including        98% Bphen as host with 2% Li metal.

Example 3 (Inventive)

An OLED device was constructed as described above for Example 1 exceptthat Step 7 was as follows:

-   -   7. A 40 nm n-doped organic layer was vacuum-deposited, including        98% of the lithium salt of        2-(2-hydroxyphenyl)-1,10-phenanthroline (Li-HPP) as host, with        2% Li metal.

Example 4 (Inventive)

An OLED device was constructed as described above for Example 1 exceptthat Step 7 was as follows:

-   -   7. A 40 nm n-doped organic layer was vacuum-deposited, including        49% Li-HPP, 49% ALQ as co-host, with 2% Li metal.

Example 5 (Inventive)

An OLED device was constructed as described above for Example 1 exceptthat Step 6a (below) was added after Step 6, and Step 7 was as follows:

-   -   6a. The above-prepared substrate was further treated by        vacuum-depositing a 30 nm undoped layer of PBNA.    -   7. A 10 nm n-doped organic layer was vacuum-deposited, including        98% Li-HPP as host, with 2% Li metal.

Example 6 (Inventive)

An OLED device was constructed as described above for Example 1 exceptthat Step 6a (below) was added after Step 6, and Step 7 was as follows:

-   -   6a. The above-prepared substrate was further treated by        vacuum-depositing a 20 nm undoped layer of 50% PBNA and 50%        8-quinolinolato lithium (LiQ).    -   7. A 20 nm n-doped organic layer was vacuum-deposited, including        98% Li-HPP as host, with 2% Li metal.

The results of testing these examples are shown in Table 1, below. Theinventive examples show, relative to their respective comparativeexamples, improved luminance, luminance efficiency, and lumens/watt.Most also show improved drive voltage, and some show improved powerefficiency and quantum efficiency.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

PARTS LIST

-   -   10 OLED device    -   15 OLED device    -   20 substrate    -   30 anode    -   35 hole-injecting layer    -   38 electron-accepting layer    -   40 hole-transporting layer    -   45 hole-transporting layer    -   50 y yellow light-emitting layer    -   50 b blue light-emitting layer    -   51 r red light-emitting layer    -   51 g green light-emitting layer    -   51 b blue light-emitting layer    -   55 electron-transporting layer    -   65 undoped layer    -   70 electroluminescent unit    -   75 electroluminescent unit    -   80 intermediate connector    -   85 intermediate connector    -   90 cathode    -   95 n-doped organic layer

1. A tandem OLED device comprising: (a) an anode; (b) a cathode; (c) atleast two electroluminescent units disposed between the anode and thecathode, wherein each of the electroluminescent units includes at leastone hole-transporting layer and one organic light-emitting layer; and(d) an intermediate connector disposed between adjacentelectroluminescent units, wherein the intermediate connector includes ann-doped organic layer and an electron-accepting layer, theelectron-accepting layer being disposed closer to the cathode than then-doped organic layer, wherein the n-doped organic layer includes analkali metal and an organic alkali metal complex, and wherein theintermediate connector further includes an undoped layer in contact withthe n-doped layer on the side closer to the anode.
 2. The tandem OLEDdevice of claim 1 wherein the organic alkali metal complex includes aphenanthroline derivative.
 3. The tandem OLED device of claim 2 whereinthe alkali metal is lithium.
 4. The tandem OLED device of claim 3wherein the organic alkali metal complex includes lithium.
 5. The tandemOLED device of claim 4 wherein the n-doped organic layer furtherincludes an uncharged phenanthroline derivative.
 6. The tandem OLEDdevice of claim 1 wherein the organic alkali metal complex includes achemical compound according to the following formula:


7. The tandem OLED device of claim 1 wherein the undoped layer includesan anthracene derivative.
 8. The tandem OLED device of claim 7 whereinthe undoped layer further includes an organic alkali metal complex. 9.The tandem OLED device of claim 8 wherein the organic alkali metalcomplex in the undoped layer is a salt of hydroxyquinoline.
 10. Thetandem OLED of claim 1 wherein the electron-accepting layer includes ahexaazatriphenylene derivative.